Polyborates anions

The apparent acid strength of boric acid is increased both by strong electrolytes that modify the stmcture and activity of the solvent water and by reagents that form complexes with B(OH) 4 and/or polyborate anions. More than one mechanism may be operative when salts of metal ions are involved. In the presence of excess calcium chloride the strength of boric acid becomes comparable to that of carboxyUc acids, and such solutions maybe titrated using strong base to a sharp phenolphthalein end point. Normally titrations of boric acid are carried out following addition of mannitol or sorbitol, which form stable chelate complexes with B(OH) 4 in a manner typical of polyhydroxy compounds. EquiUbria of the type ... 

In dilute aqueous solutions, the boric acid molecule remains undissociated B(0H)3 but in concentrated solutions, B(OH)4 and complex polyborate anions, such as B303(0H)4 are formed. Reactions with fluoride ion produce fluoroborates, BF(OH)3, BF2(OH)2, BFsCOH), BF4, and B3O3F63 in stepwise sequence. It forms similar polyions with amides and borates. 

The polyborate oxoanions can be regarded as acid anhydrides, formed by elimination of water following nucleophilic attack of a B-OH oxygen on a trigonal boron center. The polyborate anions contain one or more tetrahedral boron centers and have a negative charge resulting from acceptance of the OH ion that is shared by boron atoms within the polyborate. Polyborate... 

The structure of ulexite [26] is based on the isolated pentaborate trianion shown in Fig. 21. This polyborate anion is associated with one crystallograph-ically unique 6-coordinate Na" cation and one unique 8-coordinate Ca " cation. The cations form chains by sharing coordinated oxygen, and these chains link the polyanions together into parallel sheets. These sheets are associated by coordination with interstitial Ca " cations and hydrogen-bonding. 

Figure 5 Polyhedral representation of the polyborate anion in the compound SrLi(B305)3...

It is important to note that the presence of polyborate anions, as shown in Fignre 2, is only significant in relatively concentrated solutions. This is relevant to the enviromnental and biological aspects of boron chemistry, since boron is present in both natural waters and biological systems at low concentrations. Under these conditions, only B(OH)3 and B(0H)4 are significant species and at near neutral pH, the concentration of the B(OH)4 anion is minor. Ocean waters contain an average boron concentration of about 4.6 g p,g g , which is almost entirely present in the form of naturally occurring boric acid. Boron in plants and animals is also present mainly as boric acid, even if the dietary source of boron is a borate salt. 

Figure 4 Polyhedral representation of the polyborate anion in the compound LlBsOs. BO4 tetrahedra are lightly shaded, and BO3 triangles are deeply shaded, here, and in Figures 5-7...

With increasing concentration of boric acid (> 0.025 M) the hydration-dissociation — equilibrium is shifted towards free B(OH)3 molecules, which associate progressively. These macromolecules can be regarded as polyborate anions (J4). Some polynuclear species, B303(0H)4, B303(0H)5" and the very stable tetraborate ion, which is the most predominant dinegative ion in sodium borate solutions, have been detected (Fig. 3). 

Dale, J. The stereochemistry of polyborate anions and of borate complexes of diols and certain polyols. J. Chem. Soc. 922 (1961). 

Boiling water oxidizes [BH4]" to a mixture containing various polyborate anions, simplified here, while releasing H2 gas ... 

Depending on the pH, the cation present, and the temperature, a variety of hydrated polyborates can appear as saturating solid phases [10], In fact, many hydrated polyborates occur naturally [10]. The structures of polyborate anions in solution appear to conform to a list of rules formulated by Edwards and Ross [182] by analogy to the structures of crystalline hydrated polyborates ... 

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